View Full Version : Combi long and short eye relief loupes

Lee Christopher
8-Jun-2009, 20:33
I came across a stock of (to me) potentially interesting loupes.

Straight off - nothing razzle dazzle about them - regular straight-forward plastic loupes but here's what I found:

I'm short-sighted with fairly moderate-heavy correction of 600 deg.

When I tried this loupe (10x ... I know, 10x is too strong for lf focusing work but I bought it for other purposes), I found that I could actually use it two ways: with or without my glasses!

With glasses and it becomes a long eye relief loupe with the loupe against the GG and I can have my face anywhere from a few inches up to more than 2 feet away, and everything's sharp.

Without glasses and I could place my eye from about 2 inches away, right up to the loupe and everything is razor sharp.

I tried my 7x Horseman loupe and while it offers me both short and long eye relief, it only works with my glasses on either way.

Has anyone wearing spectacles (myopic) every found their loupes working the way I mentioned without glasses or has the difference in magnification have something to do with it?

Emmanuel BIGLER
9-Jun-2009, 02:32
can have my face anywhere from a few inches up to more than 2 feet away, and everything's sharp.

You are a happy short-sighted man since you do not suffer from astigmatism. And you probably do not suffer from presbytia as well.
I am not as happy as you and I need to keep my glass on when using a loupe... suffering from astigmatism and presbytia ;)

I do not have any precise clue for what you observe, but I hope you'll forgive me to mention here some general properties of loupes, an optical instrument that actually works in a very counter-intuitive way.

There are basically two issues in the question you describe like in any question related to optical instruments:
- issue #1 is : seing sharp;
- issue #2 is : seing a wide field, as wide as possible.

Regarding issue #1, the basic operating principle of a loupe is to send the exit image to infinity when the object is located at the object-side focal point of the loupe. Doing so, if yourself can see sharp at infinity, you can place your head as far as you wish from the loupe, you'll always see sharp with almost the same magnification !
Infinity plus a few feet is always infinity, the image cannot be further away than infinity ;)
The fact that we do not perceive actually where the exit image is located, makes a loupe a very non-intuitive optical system.
In a view camera, the image is on the ground glass, you can touch it.
In a loupe, you cannot "touch" the image, this makes the situation very strange indeed.

In fact there is a certain tolerance regardin the right positioning of the loupe of course, usually it is considered reasonable to send the image to about 3 feet (one metre) away in front of the loupe; anybody with a a proper dioptric correction is supposed to see things sharp at 3 feet in front.
Due to the possible fluctuations and range of accommodation of your eye, you can see sharp any image located between, say 20-30cm cm (8 to 12 inches) up to infinity. Your eye works as an auto-focus system, you cannot know where the image is actually located. If by bad luck the image is located at 10 cm / 4 inches, your eye might get strained by desperately trying to accommodate.
If the image was a real object like a newspaper, no question, you read at the best comfortable distance, but this is not automatically adjusted when you look at the virtual image behind a loupe.

Regarding issue #2, what usually occur with low-cost optical eye-pieces or loupes is that their eye relief is short, i.e. you loose the field very fast when looking through it at a distance of even a few inches away. This issue is decorrelated from the shaprness issue but when the field is minuscule, the loupe is simply useless.

So it is difficult to know what happens in your particular loupes, those are compound systems and nobody except the designers know actually where the exit pupil is located.
In long eye-relief eye-pieces or loupes, the exit pupil is located about one inch behind the exit lens element, so that you can look comfortably with your eye-glasses on, the idea beeing to place the eya as close as possible to the exit pupil in order to see the maximum visible field.
It depends also on the diameter of the lens elements in the loupe, this commands the diameter and location of the exit pupil, hence the amount of field you can actually see through the loupe for various positions of your head.

Well this long post does not give precise answers to your questions, but I hope that you'll find useful information to better understand how your loupes operate.

Lee Christopher
9-Jun-2009, 04:21
Emmanuel, that is a very detailed and interesting reply!

I have moderately severe astigmatism (on two planes if I remember correctly) which was why fitting my eyes for contact lenses, even the thoric soft ones was a chore; they never stayed in place and I finally gave on on contacts.

Now I am even more confused. Why is it I can go from about 2-3 inches right up to the little loupe without glasses and everything's sharp, whereas with my Horseman, at the same distance just described, I need my glasses if not everything's blurred.

Can I send you my little plastic loupe to play with and tell me what you find out? You don't have to return it after that. Think of it as a little gift for taking the time to explain all that to me.

I'm also going out to get the lower powered little loupes (I guess it's easier to just name them such) 4x and 6x to see if I get the same experience as the 10x.

Emmanuel BIGLER
9-Jun-2009, 07:07
Christopher, feel free to send may any loupe, I accept with pleasure but only at one non-negociable condition, that I can send it back freely to you afterwards !
(you can contact me through the forum's message system)